What is a good test as to whether fruits or vegetables are poisonous in the Swiss Family Robinson?

In the novel "Swiss Family Robinson," the characters use a simple test to determine if a fruit or vegetable is poisonous. They first rub a small amount of the substance on their lips to check for any immediate adverse reactions. If there are no negative effects after a period of time, they then place a small amount on their tongue to further test for toxicity. This method allows them to assess the safety of unfamiliar plants before consuming them.